Traveling wave amplifier tube



Sept. 11, 1962 F. E. PASCHKE TRAVELING WAVE AMPLIFIER TUBE 2 Sheets-Sheet 1 Filed Aug. 5, 1958 m? E 2 n m Sept. 11, 1962 F. E. PASCHKE TRAVELING WAVE AMPLIFIER TUBE 2 Sheets-Sheet 2 Filed Aug. 5, 1958 INVENTOR. FRITZ E. FAscz-ma.

United States Patent 3,054,018 TRAVELING WAVE AMPLIFIER TUBE Fritz E. Paschke, New Brunswick, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Aug. 5, 1958, Ser. No. 753,234 17 Claims. (Cl. 315-3.6)

This invention relates to improved traveling wave amplifier tubes and particularly to magnetron amplifier tubes.

A traveling Wave magnetron amplifier tube has been suggested that comprises an elongated, angularly-periodic signal wave propagating structure adapted to propagate signal waves therealong in axial and transverse directions and having a predetermined axial phase velocity, and means for projecting a beam of spiralling electrons along the wave propagating structure in wave interaction relation therewith and in a radial direct-current electric field, with'the electrons having an axial velocity component substantially less than the predetermined axial phase velocity of the structure. The electrons in the beam are projected in spiral paths such that at substantially all points in its path each electron is subjected to a substantially constant-phase electric field produced 'by' a given space harmonic of the signal Waves propagated along the structure.

In order to have interaction between an electron beam and a signal wave, the beam velocity must be in the same direction as, and suitably matched to, the phase velocity of the signal wave with which interaction is desired.

In a traveling wave magnetron amplifier tube, it is desirable to use a dispersive wave propagating structure, since such a structure is adapted to propagate signal waves with a lower group velocity than in a non-dispersive structure and thus lends itself to better coupling with the electron beam of the tube. However, as is known, dispersive wave propagating structures usually lead to narrow bandwidth operation.

Accordingly, one object of this invention is the provision of an improved traveling wave amplifier tube having a dispersive wave propagating structure wherein the tube is capable of uniformly amplifying. relatively high frequency signal waves over a relatively-wide frequency range.

This object is achieved in an electron tube having a novel and improved wave propagation structure which is periodic in two directions. Preferably, the periodicity of the structure is such that a change in phase velocityversus-frequency characteristic in one of the directions is compensated by an opposite change in phase velocityversus-frequency characteristic in the other direction, to give resultant wide band width.

Another object of this invention is the provision of such a biperiodic wave propagation structure wherein energy propagation or power flow is precluded in one of the two directions of periodicity so as to prevent operation of the tube in any mode other than a single desired one.

Another object of this invention is the provision of novel and improved coupler apparatus especially useful in conjunction with the novel biperiodic wave propagating structure according to this invention. 7

According to a preferred embodiment of the invention, an electron tube makes use of cylindrical geometry for effectively propagating traveling waves both in a circumferential direction and in an axial direction. This tube includes circumferential and axial wave propagating means in the form of periodic delay lines. A hollow beam of spiralling electrons is injected axially into an elongated annular space between two concentric cylindrical conductive structures in an axial magnetic field.

it is continuously subjected to an electric field component of substantially constant phase. Traveling waves propagated axially and circumferentially along the line produce space harmonic waves effectively mo'ving helically around the array of fingers, with the spiralling electrons in the beam interacting with a constant phase of the electric field of a given one of the space harmonics to pro;- duce amplification of the signal. This amplification is produced by the following phenomena. First, the electric fields of the space harmonic on the transmission line produce electron bunching in the direction of electron travel. In the presence of a radio frequency signal, the electron bunches are accelerated radially toward the transmission line by the radial direct-current electric field, which thereby delivers energy to the electrons. In turn, the electrons deliver energy to the transmission line by inducing radio frequency currents in the fingered cond-uctors, which energy appears as amplified radio frequency energy of the output signal wave. This latter phenomenon is discussed by R. R. Warnecke et al. in the article The Magnetron Type Traveling Wave Am.- plifier Tube, Proceedings of IRE, volume 38, 1950', pages In the drawing: FIG. 1 is a longitudinal section view of a tube illustrating a preferred embodiment of my invention;

FIGS. 2a and 2b are plan and side elevation-view respectively of an elemental part of the dispersive wave propagating structure of the tube of FIG. 1 according to a preferred fabrication of my invention;

FIGS'. 3a and 3b are plan and side elevation views respectively of another elemental part of the dispersive wave propagating structure of the tube of FIG. I according to the preferred fabrication of my invention;

FIG. 4 is a diagram illustrating an aspect of an operation of a traveling wave magnetron tube of the invention; and V FIGS. 5a and 5b are plan and side elevation views respectively of novelcoupler apparatus useful in conjunction with the dispersive wave propagating structure of FIG. 1.

" In FIG 1 there is shown a magnetron traveling wave amplifier tube incorporating a preferred embodiment of my invention. A stacked electron gun structure 12 together with end plates 14 and 16 of input and output coupler cavities, an electron collector electrode 18, and "a tubular member .20 form avacuum envelope for the tube. A relatively dense hollow stream of electrons 22 is emitted from a thermionically electron emissive surface 24 of an annular cathode 26. The cathode 26 is provided with a heater (not shown). Electrons 22 from the cathode 26 are accelerated through an annular grid 28 due to a positive bias on it. A solenoid coil 30 around the tube produces a time-constant magnetic field parallel to the axis of the tube. After the electrons 22 pass through the grid 28 they are subjected to, a radial direct current electric field between a first cylindrical center conductor 32 positioned around the axis 'of the tube and a longitudinally extending hollow cylindrical electrode 34spaced around and in coaxial relation with the center conductor 32. The center con- 'ductor 32 and the other electrodes of the tube are all made of a magnetically transparent material to maintain the axial magnetic field substantially free of distortion. The hollow electrode 34 is biased at. a higher positive potential than the center conductor 32 so as to establish the radial electric field aforementioned which accelerates the electrons, after their passage through the grid 28, outwardly and across the axially-extending magnetic flux lines produced by the coil 30. As the electrons move outwardly by virtue of the radial electric field,

the magnetic flux lines cause the electrons to acquire spiral motion. The electrons then enter an annular interaction space between a second cylindrical center conductor 36 and a hollow dispersive wave propagation structure or transmission line 38 comprising an array of fingers 40 extending radially inward toward the center conductor 36 and disposed in both circumferential and axial rows. The fingered cylindrical structure 38 constitutes the anode of the tube. A direct current voltage source, indicated schematically by a battery 42, is connected between the center conductors 32 and 36 and the dispersive line 38 to establish the radial direct current electric field in the annular space therebetween. This radial electric field subjects the spiraling electrons to a radially outward force which together with the centrifugal force on the electrons opposes the inward force exerted on the electrons by the axial magnetic field and maintains the electrons in the desired spiral paths as they interact with the signal wave propagated along the array of fingers 40. The electrons drift axially of the tube, with the spiral motion described, toward the collecting-electrode 18' positioned at the end of the tube remote from the cathode 26. Actually, some of the electrons will be intercepted by the finger members 40 as they move outwardly under the influence of the radial electric field.

In order to aid in the assembly of the tube, the second center conductor 36 may be supported by the collecting electrode 18 at one end of the tube by means of a ceramic ring 44 sealed to a rod 46 extending from the second center conductor 36. The ring 44 may be slidable with respect to the collecting electrode 18 to permit expansion and contraction of the tube parts during normal operation of the tube.

In the operation of the tube, electromagnetic signal waves are fed to the dispersive wave propagation'line 38 through an input line 48 and an inputcoupler cavity arrangement 49. The input coupler cavity 49 is defined by the end plate 14, a novel fingered coupler disk 50, the center conductor 32 and 36, and an endwall 52 on the first center conductor 32 inside the second center conductor 36'. If desired, this cavity can be adjusted when the tube is assembled by a slidable adjustment of the first center conductor 32 to within the second center conductor 36.

The electric fields of the signal waves fed to the dispersive wave propagation line 38 give rise to an infinite set of space harmonics, one of which is the space harmonic with which the spiraling electrons are in energy transfer relation. The electrons are subjected to a substantially constant phase electric field produced by that space harmonic, which results in bunching of the electrons.

.The electron bunches, in turn, interact with the electric .line 38 by a cavity coupler arrangement 55 similar to that employed for the input coupler 49. The dispersive transmission line 38 and the input and output coupler arrangements are described more fully with reference to FIGS. 24 through b.

The dispersive transmission line 38 is illustrated in FIG. 1 as an integral structure. However, as will be described with reference to FIGS. Za-Bb, the line 38 is, according to a preferred fabrication, made up of a series of stacked parts. Such preferred construction is purely for the purpose of simplicity of fabrication and has no electrical consideration whatsoever. In FIG. 1 the transmission line 38 is shown to comprise a hollow metallic cylinder having internally thereof a multiplicity of conductive'fingers 40 which extend radially inward and which are arranged in both circumferential and axial rows. Although not shown in FIG. 1, an important feature of the fingered transmission line. 38, which will become apparent from the description of FIGS. 2a-3b, is the fact that the fingers 40 are webbed along axial rows for a part of their length from their base toward their inner ends. Such a construction, in effect, results in a shorter finger length being presented to wave propagation in an axial direction than the finger length presented to wave propagation in a circumferential direction. Reference to FIGS. 211-317 will provide a clearer appreciation of this feature.

FIGS. 2a-3b illustrate two different kinds of elemental parts to be stacked together to fabricate the dispersive transmission line 38 according to preferred practice. As shown in FIGS. 2a and 2b, one of these parts is an annular disk 56 comprising a thin, metal member having a multiplicity of inwardly, radially extending fingers 58 uniformly spaced in a circular array. The fingers 58 are provided by a corresponding multiplicity of radial slots 60' cut through the disk 56 from its inner periphery toward its outer periphery. FIGS. 3a and 3b illustrate the other elemental part comprising an annular metallic disk 62 used in conjunction with the annular disk 56 in the preferred fabrication of the dispersive transmission line 38. The disk 62 is formed with radial slots 63 providing fingers 64 that are shorter than are the fingers 58 of disk 56, e.g., approximately one-third as long.

To fabricate the dispersive transmission line 38 a multiplicity of the annular disks 56 and 62 are stacked alternately in coaxial array with the slots and fingers in alignment. Thus, it will be appreciated that the fingers 58 serve as the fingers 40 of the dispersive transmission line 38 as illustrated in FIG. 1, and the fingers 64 of disks 62 serve only as webs between adjacent fingers 40 along axial rows thereof.

With the dispersive transmission line 38 so fabricated, the fingers 40 are webbed along the axial rows thereof but are full depth along circumferentially rows. Therefore, as previously noted, the finger length presented to axially propagated electromagnetic waves is considerably shorter than the finger length presented to circumfererr tial-ly propagated electromagnetic waves.

In this arrangement, the axially-aligned slots 60 and 63 form relatively-deep longitudinal grooves 65 in the cylindrical structure 38', and the spaces between the axially-spaced fingers 58 not occupied by the fingers 64 form relatively-shallow transverse or circumferential grooves 66 in the structure.

The tube of FIG. 1 is designed to operate in a preselected specific band of frequencies. In accordance with the invention, the transmission line structure is designed to operate only in the stop band for circumferential wave energy propagation wherein the electrical length of the fingers 58 is greater than M4 and less than A/Z'at each frequency in the operating band. 'According-ly, the full length finger 58 presented to circumferentially propagated waves can be made approximately in length with respect to the center frequency of the band. On the other hand, in order 'to propagate wave energy in the axial direction; the'electrical length of the finger 58, as shortened by the webbing finger 64 and as presented to axially propagated waves, must be either equal to or less than x/4'over the operating band, and hence,

less than M4 with respect to the center'freque'ncy. In other words, the depth of the longitudinal grooves 65 is greater than M4 and less than )\/2, and the depth of the circumferential grooves 66 is equal to or less than )t/ 4, at each frequency in the operating hand. Now, considering the composite dispersive transmission line 38 in conjunction with the center cylindrical electrode 38 of FIG. 1, it will be appreciated that periodic transmission lines of the vane or comb type are provided both in axial and circumferential directions, and that, moreover, these periodic delay lines are provided with folded elements having lengths of equal to or less than M4 and between M4 and M2 respectively. From well-known wave propagation principles, it will be appreciated that a wave propagating dispersive transmission line operating in the pass band is provided along an axial direction by virtue of the electrical length of the fingers being equal to or less than M4, while on the other hand, a non-propagating periodic dispersive structure operating in the stop band is provided in the circumferential direction by virtue of the electrical length of the fingers being between M4 and 7\/2 in length. Therefore (electromagnetic energy applied to one end of the composite dispersive transmission line 38 will be propagated axially along the line to the other end thereof. At the same time, no circumferential energy propagation will be permitted within the operating band width of the tube.

Since circumferential energy propagation is prevented, the phase shift between adjacent fingers in circumferential rows can be maintained equal to 1r irrespective of frequency. Therefore, by energizing only alternate fingers 58 of a disk 56 at one end of the dispersive line 33, adjacent circumferential fingers are 180 out of phase, and -r mode operation assured. In such operation, energy is propagated axially along each axial row of fingers 40 and, although no energy is capable of circumferential propagation, the effect of circumferential wave propagation (an effective circumferential propagation of a predetermined phase of the signal input) can be achieved by virtue of the following phenomenon.

At any given instant of time, the phase of the signal on one axial row of fingers 40 is farther or less advanced than the corresponding phase of the signal on either adjacent axial row of fingers. The spiralling electrons have a path of such pitch that as each electron goes from a given position between the fingers in one pair of axial rows to a second position between the fin gers of the adjacent pair of axial rows, it is subjected to an electric field component of the same phase as at the first position. Of course, the electron does not see the same half waves at the second position as it saw at its first position, but it sees instead the field of an analogous pair of half waves. Similarly, when the electron arrives at a position adjacent the next pair .of axial rows of fingers, it sees the field of another pair of half waves which appear like the first pair. This continues along the spiral path of the given electron. Thus, a given electron will see what appears to be the field of the same wave during its spiral travel. What the electron really sees is the field of the fundamental space harmonic wave. While given electrons will see the peak amplitude of the field of the space harmonic during their spiral travel, as described above, other electrons in other portions of the beam will see other amplitudes of the field of the same space harmonic, and hence, some electrons will be continuously accelerated while others are continuously decelerated, which will result in circumferential bunching of the electrons. The electron bunches thus formed interact with the fields of the signal wave to increase the amplitude of the signal.

In accordance with the invention, a wave propagation structure is provided which is dispersive but which at the same time has a wide band width by virtue of the fact that the phase velocity along a helical path of the struc ture remains substantially constant through changes in frequency of signal waves'propagated along the structure. This is accomplished by providing a wave propagation structure which is adapted to simultaneously effectively propagate signal waves in 'two different directions, i.e., in both axial and circumferential directions. According to well known microwave principles, a forward wave propagated in the axial direction will experience a decrease of phase velocity with an increase in frequency. However, this decrease is compensated for by an increase in the phase velocity of the wave effectively propagated in the circumferential direction.

The fact that the effective circumferential propagation increases inphase velocity with an increase in frequency is shown by the well known equation where w=angular frequency, p=angular phase velocity, b=phase shift per length l, and l=distance between adjacent axial rows of fingers. Remembering that in the dispersive transmission line 38 phase shift between adjacent axial rows, or ,0, is constant, 1r mode operation existing, and distance between axial rows, or I, is also constant it follows that is a constant. This means therefore that as frequency (0)) increases, the phase velocity (p) must increase correspondingly.

The biperiodic transmission line according to the invention can be shown to fulfill the previously stated quality of compensating for a decrease in axial phase velocity by an increase in circumferential phase velocity. In order for broad band operation to exist it is necessary that through the normal frequency range there be atleast one spiral phase velocity which remains substantially constant irrespective of the fact that the component axial and circumferential phase velocities vary with frequency. Then, the electron beam can be so adjusted that it will correspond to this constant phase velocity thus maintaining the required synchronous relation between the elec-. tron beam and the spiraling traveling waves. Such a condition for synchronism can be expressed by the equation v 1),, FY p 1 where v is the velocity component of an electron in a circumferential direction, p is the phase velocity of the propagated wave in the circumferential direction, v is the velocity component of the same electron in the axial direction, and p is the phase velocity'of the propagated wave in the axial direction. For practical purposes the ideal biperiodic structure should be such the Equation 2 is satisfied. even with the additional requirement that i and v remain substantially constant over the frequency. range. This permits the electron beam to be established alonga given spiral path and to be maintained there as is necessaryv according to practical tube operation. According to the inherent characteristics of my invention, Equation 2 is so satisfied even with this additional requirement.

' In .FIG. 4 this compensation is illustrated graphically wherein point 0 is the point of origin of signal waves whose phase velocities are indicated in Z and Y direc tions. Fora given frequency h, the phase velocity is p in the axial direction and p in-the circumferential direction; Accordingly, the line BC is a line of constant phase, being drawn between p and p of the same phase. The line BC may be thought of as representinga wave front moving away from point 0. For a second frequency f the axial phase velocity is decreased to, and represented by, p and the circumferential phase velocity increased to, and represented by, p Thus, the line DB of constant phase defined by f intersects BC at point H. According to the inherent character of the invention, a third frequency f has axial and circumferential phase velocities represented by p and p which define the line FG of constant phase for i and which also intersects constant phase lines BC and DE substantially at the same point H. Thus, an electon beam projected at an angle of spiral 6 according to vector v will be maintained in desired synchronous relation with the spiral phase velocity throughout the normal frequency range where Equation 2 holds true.

In order to excite the dispersive transmission line 38 in accordance with the desired 1r mode operation as described above, the novel coupler device as indicated by numeral 50 in FIG. 1 is provided according to my invention. FIG. a shows theannular disk 50 to be similar to the disk 56 of FIGS. 20! and 2b mainly with the exception that exactly /2 as many fingers 68 are provided in the disk 50 as fingers 58 are provided in disk 56. The coupler disk 50 is coaxially stacked with the disks 56 and 62 with adjacent fingers 68 of the coupler disk 50 centrally overlying alternate fingers 58 of the adjacent elemental part disk 56. This alignment is indicated by the relative angular disposition of the section lines 11 in FIGS. 2a, 3a, and 5a. With such an arrangement, electromagnetic energy injected into the input coupler cavity from the transmission line 48 is applied as indentically phased versions of the input signal to circumferentially alternate fingers 40 of the dispersive transmission line 38. Accordingly, the dispersive transmission line 38 is energized in the desired 1r mode with circumferentially adjacent fingers 58 of any given disk 56 being at any given time energized 180 out of phases with each other.

The input coupler cavity has been previously described as being defined by the coupler disk 50, (the end plate 14, center conductors 32 and 36, and the end wall 52. It can be seen by referring to FIG. 1 that the end plate 14 forming the input coupler cavity is provided with a U-s-haped channel 70 extending circumferentially therein. The total electrical length of the channel 70 is made to be approximately equal to M4 for the operating band width center frequency and, as such, presents an infinite impedance across the gap 72 to prevent leakage of the RF energy from the input coupler cavity into the electron gun region.

The output coupler cavity is defined by structure similar to that of the input coupler cavity and includes the end plate 16 and a coupler disk 50 identical to the coupler disk used for the input coupler cavity. Like the input coupler end plate :14, the output coupler end plate 16 is also provided with a channel approximately M4 in electrical length extending circumferentially therein. In similar manner, this channel in the end plate 16' prevents leakage of RF energy from the output coupler cavity into the region of the collector 18.

While the tube of FIG. 1 has been described withrespect to a transmission line in which the line is positioned around a center conductor, it will be realized that the transmission line may be disposed within a hollow cylindrical conductor with a beam of spiraling electrons traveling between the transmission line and the conductor. This inverted structure requires that the angular velocity component of the electrons be directed such that the radial forces on an electron due to the angular velocity component of the electron and to the magnetic field are both directed radially outward, balancing the electric field force.

In the tube described, some of the potential energy of the electron beam derived from the transverse direct current electric field is given up by the electrons as amplification of the signal wave, the velocity (and kinetic energy) of the electrons remaining substantially constant. This is to be distinguished from the usual linear traveling wave tube where the energy used for amplification, is the kineticenergy of the electrons which are or slowed down as they give up energy. The transverse direct-current electric field causes the electrons to move transversely toward the transmission line as they give up potential energy to the radio frequency electric field while retaining their original velocity. Since, as has been described above, the dispersion of the structure is Such that a change in phase velocity-versus-frequency characteristic of the line in one of the two directions of wave propagation is compensated by an opposite change in phase velocity-versus-frequency characteristic of the line in the other direction, the electrons are in synchronism with a constant phase of a space harmonic of the signal for any desired frequency within the normal signal wave frequency range of the tube.

The biperiodic wave transmission line described has special utility in the millimeter wave region where the small wave lengths encountered give rise to a need for relatively small size wave transmission lines. As is known, in order to keep the dimensions of a line to a reasonably large size for mechanical reasons, the phase velocity of the line must be relatively high; the phase velocity and the size of a line being proportional at any given frequency. When high transmission line phase velocities are used, high electron beam velocities are needed. But high beam velocities are undesirable. When a biperiodic transmission line is used, the phase velocity of the line in both of the directions of periodicity can be relatively high while the phase velocity of the fundamental space harmonic seen by the beam is relatively low. For example, if the phase velocities in two directions apart are the same, and the angle of the electron beam with respect to the line is 45, the phase velocity seen by the beam is 2 /2 times the phase velocity of the line in either of the two directions separately. Consequently, a line according to the invention can propagate a fundamental space harmonic with a velocity v2/2 times lower than that of waves propagated in either of the two directions of periodicity and the electron beam voltage needed is /2 that needed when either of the two directions of periodicity is used alone.

From the foregoing it is seen that the tube of the invention makes use of a dispersive wave propagating structure for achieving good coupling with an electron beam of the tube and at the same time is capable of uniformly amplifying relatively high frequency signal waves over a frequency range which is wider than that of previous traveling wave magnetron amplifier tubes. Furthermore, While the tube of the invention is especially useful for the amplification of amplitude modulated waves, it will be realized that the tube is also advantageous for the amplification of phase or frequency modulated signal-waves over a wide frequency range.

It will also be appreciated that the biperiodic transmission line of the invention is especially suited for high power operation. Unlike prior art transmission line designs, an increase in physical size of the line of my invention, in order to enable dissipation of large amounts of power, will not affect the line electrically so as to bring on undesired operational modes. This, of course, results from the inherent quality of the line being nonpropagative of energy in the circumferential direction and thereby assuring 1r mode operation irrespective of frequency. By contrast, in prior art designs where overall physical size is closely interrelated with frequency this is not so.

Although the biperiodic transmission line according to my invention has been described as incorporated in a magnetron type traveling wave tube, it can as advantageously be used in O-type traveling wave tubes. To this end the center conductor 36 can be omitted and the radial electrical field dispensed with. As such, magnetron action of providing potential energy to the electrons that they may give up for amplification is not utilized. However, operation using either spiral flow or a parallel flow 9 electron beam will still obtain the desirable propagation quality described with respect to the tube of FIG. 1 wherein undesirable circumferential modes are prevented.

I claim:

1. A biperiodic dispersive transmission line adapted for operation over a predetermined frequency range comprising conductive means for propagating waves and wave energy in said range on said line in one direction, and conductive means for preventing propagation of wave energy while permitting propagation of waves in said range on said line in a second direction transverse to said one direction.

2. A traveling wave tube comprising the combination of a periodic dispersive transmission line according to claim 1 with electron gun means for projecting electrons along said line at an angle to said two directions for interaction with a selected space harmonic of said waves.

3. A biperiodic dispersive cylindrical transmission line adapted for operation over a predetermined frequency range comprising conductive means for propagating waves and wave energy in said range in axial direction on said line, and conductive means for preventing propagation of wave energy while permitting propagation of waves in said range in circumferential direction on said line.

4. A traveling wave tube adapted for operation over a predetermined range of frequencies including a biperiodic dispersive cylindrical transmission structure comprising conductive means for propagating waves and wave energy in said range in axial direction thereon and conductive means for preventing propagation of wave energy while permitting propagation of waves in said range in circumferential direction thereon, and electron gun means for projecting electrons along said structure in spiral paths about the longitudinal axis thereof for interaction with a selected space harmonic of said waves.

5. A biperiodic dispersive transmission line adapted for operation over a predetermined frequency range, comprising a conductive member including a plurality of fingers extending normal to one surface thereof and disposed in an array of longitudinal and transverse rows, said conductor being adapted to effectively propagate electromagnetic waves in said range in both longitudinal and transverse directions, said conducting member comprising means for propagating wave energy in one of said direc tions and preventing propagation of wave energy in the other direction at each frequency in said range.

6. A traveling wave tube comprising the combination of a biperiodic dispersive transmission line according to claim 5 with electron gun means for projecting electrons along said line at an angle to said two directions for interaction with a selected space harmonic of said waves.

7. A biperiodic dispersive transmission line adapted for operation over a predetermined frequency range comprising an elongated hollow cylindrical conductor including a plurality of fingers internally thereof extending ra dially inward and disposed in an array of axial and circumferential rows, said conductor being adapted to effectively propagate electromagnetic waves in said range in both axial and circumferential directions and comprising means for propagating wave energy in axial directions and preventing propagation of wave energy in circumferential directions at each frequency in said range.

8. A biperiodic dispersive transmission line according to claim 7, wherein said means comprises conductive Webbing disposed between and contacting the bases of axially adjacent fingers.

9. A biperiodic dispersive transmission line adapted for operation over a predetermined frequency range, comprising a conductive member having a plurality of equally spaced longitudinal grooves and a plurality of equally spaced transverse grooves in one surface thereof, the depth of said transverse grooves being equal to or less than M4 at each frequency of said range to permit propagation of wave energy along said line in a direction nor- 10 trial to the transverse grooves, the depth of 'said'longi'tudinal grooves being greater than M4 and less than M2 at each frequency of said range to permit propagation of waves while preventing propagation of wave energy in a direction normal to the longitudinal grooves.

10. A biperiodic dispersive transmission line adapted for operation over a predetermined frequency range, comprising an elongated hollow cylindrical conductor having a plurality of equally spaced circumferential grooves and a plurality of equally spaced longitudinal grooves in the inner cylindrical surface thereof forming radially extending conducting fingers disposed in a uniform array of axial and circumferential rows, the depth of said circumferential grooves being less than k/ 4 at each frequency of said range to permit propagation of wave energy along said line in a direction normal to the circumferential grooves, the depth of said longitudinal grooves being greater than )\/4 and less than M2 at each frequency of said range to permit propagation of waves while preventing propagation of Wave energy in a direction normal to the longitudinal grooves.

11. A transmission line according to claim 10 wherein the depth of said longitudinal grooves is substantially equal to at the center frequency of said range.

12. A traveling wave amplifier tube adapted for operation over a predetermined range of frequencies comprising an electron gun adapted to project a hollow beam of electrons spiralling about a common axis, a biperiodic dispersive hollow transmission line adapted to propagate waves therealong in both axial and circumferential directions, and an electron collector electrode all coaxially disposed along said axis in the order named, said transmission line including a cylindrical conductor having a plurality of conductive fingers extending radially therefrom and disposed in an array of axial and circumferential rows, and means associated with said fingers for propagating wave energy in axial directions and preventing propagation of wave energy in circumferential directions at each frequency in said range.

13. A traveling wave amplifier tube according to claim 12 wherein said means comprises conductive webs disposed between and contacting the fingers in said axial rows and extending from the bases partially along said fingers.

14. A traveling Wave amplifier tube comprising an electron gun including an annular cathode, a pair of concentric cylindrical conductors, and a collector electrode all disposed in axial alignment, said electron gun being adapted to project a hollow beam of spiralling electrons along and between said concentric cylindrical conductors in wave interaction with one thereof, said one of said cylindrical conductors including a plurality of fingers disposed in a uni-form array of axial and circumferential rows for propagating waves along said one conductor in both axial and circumferential directions, said fingers extending radially toward the other of said cylindrical conductors, said fingers being conductively joined adjacent their bases along said axial rows by conductive webs.

15. A traveling wave amplifier tube according to claim 14, further comprising means connected to said concentric conductors for establishing a radial direct-current electric field therebetween, and means for establishing a timeconstant magnetic field along the path of said beam and parallel to the axis of the tube, to obtain magnetron interaction between said electrons and said waves and electric field.

16. Microwave apparatus adapted for operation within a predetermined band of frequencies comprising a biperiodic dispersive transmission line and associated input and output coupler apparatus, said biperiodic transmission line including a hollow cylindrical conductor having a plurality of fingers internally thereof extending radially inward and disposed in a uniform array including axial and circumferential rows, said fingers of said axial rows being electrically webbed together over a part of their length adjacent their bases, the unwebbed length of said fingers being greater than M4 and less than M 2 at each frequency of said predetermined band, said input and output coupler apparatus each comprising annular cavities having one end wall thereof formed by annular fingered coupler disks, said annular fingered coupler disks each including a plurality of fingers uniformly angularly spaced and extending radially inward from the inner periphery of said annular disk, said plurality of fingers of said annular disk being equal to half the number of axial rows of fingers in said biperiodic dispersive transmission line, each of said coupler disks being disposed adjacent to an end of said biperiodic dispersive transmission line coaxially therewith with adjacent fingers of a coupler disk being centrally opposite alternate rows of fingers of said biperiodic dispersive transmission line for coupling with said biperiodic dispersive transmission line in conventional 1r mode operation.

17. A magnetron traveling wave amplifier tube comprising an electron gun, a pair of concentric cylindrical electrodes radially spaced from each other and a collector electrode, one of said concentric cylindrical electrodes including an array of fingers extending radially toward the other of said concentric cylindrical conductors and being disposed in both axial and circumferential rows with a predetermined number of fingers in each circumferential row, said fingers of said one of said concentric cylindrical conductors being partially webbed adjacent their bases along said axial rows thereof to present a shorter finger length to axial wave propagation than to circumferential wave propagation, said fingers and said webbing being of such dimensions as to present a finger length equal to or less than 8/4 to axial propagation and greater than M4 and less than M2 to circumferential propagation at each frequency in the intended operating band of said tube, a pair of coupler disks forming one end wall respectively of a pair of annular cylindrical coupling cavities, each of said coupler disks including a plurality of radially extending fingers equal to half said predetermined number, a different one of said coupler disks being disposed adjacent a different end of said one of said concentric cylindrical conductors and coaxially therewith with adjacent fingers of said coupler disks centrally overlying alternate fingers of said one of said concentric cylindrical conductors of an end circumferential row of fingers thereof, said electron gun being adapted to project a hollow beam of electrons in spiral paths along and between said pair of concentric cylindrical conductors.

References Cited in the file of this patent UNITED STATES PATENTS 2,643,353 Dewey June 23, 1953 2,683,238 Millman July 6, 1954 2,812,467 Kompfner Nov. 5, 1957 2,849,643 Mourier Aug. 26, 1958 2,858,472 Karp Oct. 28, 1958 2,888,598 Palluel May 26, 1959 

